1. Field of the Invention
The present invention generally appertains to new and novel improvements in air conditioning systems wherein a central supply of hot or cold air is distributed to remote locations as in large commercial, office-type buildings. At these remote locations there is generally a fan powered terminal unit or induction unit that is thermostatically controlled. Remote location control is essential since demand can vary significantly due to such variables as exposure and distance from an exterior wall.
2. State of the Prior Art
Fan powered variable-air-volume (VAV) terminal units have been used extensively. Ducting is provided to connect the cold or hot air supply to a plurality of terminal units. Each of these units has a mixing chamber in order to mix hot or cold air (primary air) with air originating from the zone to be conditioned (secondary air). Mounted within the terminal unit is an inlet damper or throttling valve that provides the control of primary air delivered to the unit. Also mounted within the unit is a blower or fan which causes the mixed primary and secondary air to flow from the interior of the unit through an outlet into the zone to be conditioned. A temperature sensing means is located in the air conditioned zone controlling the inlet damper and the power to the fan. The thermostatic sensing means may be electrical but more frequently in commercial installations, pneumatic systems are used. A pneumatically controlled system with these features is disclosed in U.S. Pat. No. 3,951,205. The problem with this prior art arrangement is that there is only a marginal correlation between the air flow through the throttling damper and the thermostat signal which also turns on the fan. Since the fan is usually a forward curved centrifugal wheel which has a distinct overloading characteristic on increasing air flow, if the fan is turned on while the primary air flow exceeds the design capacity for the fan, it is possible to overload the fan motor.